1. Field of the Invention
The invention relates to a transmission ratio variable device.
2. Discussion of Background
Conventionally, there has been a transmission ratio variable device that changes a rotation transmission ratio (a steering gear ratio) between input and output shafts by adding motor-driven rotation to rotation of the input shaft based on a steering operation by using a differential mechanism and transmitting resultant rotation to the output shaft. In US2008/0251311 A1, a transmission ratio variable device is described in which a nutation gear mechanism is employed as a differential mechanism. The nutation gear mechanism includes a first gear that rotates together with an input shaft, a fourth gear that rotates together with an output shaft, and a nutation gear. The nutation gear includes a second gear that meshes with the first gear, and a third gear that meshes with the fourth gear, and the nutation gear rotates about an axis that is inclined with respect to an axis of the first and fourth gears. In such a transmission ratio variable device, a good meshing state of the nutation gear mechanism is maintained by applying an axial preload to the nutation gear mechanism using an elastic member such as a spring.
In the conventional structure stated above, a deformation amount of the elastic member may vary due to an assembly error and the like, in an early stage of manufacturing of the transmission ratio variable device. As a result, the preload applied to the nutation gear mechanism may vary, and it may be difficult to maintain the good meshing state of the nutation gear mechanism. More specifically, if the preload applied to the nutation gear mechanism is too small, disengagement between the first gear and the second gear, or between the third gear and the fourth gear, that is so-called “ratcheting”, may occur when, for example, a large load is applied to the nutation gear mechanism. Meanwhile, if the preload is too large, for example, meshing friction between the gears may be increased, which may inhibit smooth operations of the nutation gear mechanism.